1. Field of the Invention
The present invention relates to a bidirectional or two-way optical transmission system designed and suited for transmission light signals of wavelengths differing from one another in up-link and down-link directions, respectively, by using a single optical fiber transmission line.
2. Description of the Prior Art
The analogue transmission system in which a semiconductor laser diode (hereinafter referred to as laser diode or LD) is employed promises to be applicable to the transmission of image information or data of industrial television, high-quality television, cable television and the like systems. However, in the transmission system in which the laser diode is used in combination with a multi-mode optical fiber, there arises a problem that so-called speckle noise and speckle distortion appear due to mutual interference among different transmission modes of laser light because of the interferential nature thereof. For this reason, the possible maximum transmission distance is limited to a range of 1 to 2 km at best in the present state of the art. In contrast, in a transmission system in which the laser diode is used in combination with a single-mode optical fiber, the speckle noise and speckle distortion mentioned above can be evaded, whereby a long distance transmission around 10 km is expected to be realized. It should however be pointed out that in the transmission system employing the single-mode optical fiber, the two-way transmission has not yet been realized at present, which can be explained by the facts that the optical multiplexer of low loss is difficult to be realized and that the transmission system has a very complicated structure and suffers from degradation in the transmission quality, low reliability and is very expensive.
FIG. 1 of the accompanying drawings shows a general arrangement of a two-way optical transmission system which is realized by using those devices and components which are commercially available at present. Referring to FIG. 1, it is assumed that the path of a light signal (of wavelength .lambda..sub.1) propagating through an optical fiber 7 in the direction indicated by an arrow 8 is referred to as an up-link, while the path of a light signal (of wavelength .lambda..sub.2) traveling in the direction indicated by an arrow 8' is termed the down-link. In the up-link circuit, a signal applied to information or data input terminal 15 is supplied to a laser diode drive circuit 14 (hereinafter also referred to as LD drive circuit). A laser diode or LD 1 is driven by the output signal of the LD drive circuit 14. The oscillation wavelength of the LD 1 is represented by .lambda..sub.1. The laser light emitted by the LD 1 passes through a spherical lens 2 and a graded index (GRIN) rod lens 3 (having a length of about 1/4 pitch, where one pitch corresponds to a period of a meandering optical path within the rod lens) and enters an optical fiber 4. Having traveled in the direction indicated by an arrow 5, the laser light reaches an optical multiplexer 6. The output light signal from the optical multiplexer 6 travels through the optical fiber 7 in the direction indicated by the arrow 8 to reach a photoelectric or optoelectric receiver 11 after having passed through an optical multiplexer 6' and an optical fiber 9. The electric output signal produced by the receiver 11 is, after having been amplified by an amplifier 12, demodulated by a demodulator 13, whereby the original information or data signal is reproduced. In the case of the down-link circuit, a signal applied to a data input terminal 15' is applied to a laser diode drive circuit 14', the output signal of which drives a laser diode or LD 1' which then oscillates at the wavelength .lambda..sub.2. The laser light emitted by the LD 1' travels through a spherical lens 2', a graded index (GRIN) type rod lens 3' (having a length of about 1/4 pitch), an optical fiber 4', an optical multiplexer 6', the optical fiber 7 and then the optical multiplexer 6 in this order and reaches a photoelectric receiver 11' through an optical fiber 9', as indicated by an arrow 10'. A corresponding electric signal produced by the receiver 11' is amplified by an amplifier 12' and subsequently demodulated by a demodulator 13' to the original data signal. As will be seen, the transmission system described above requires a great number of devices or components. Among them, the optical multiplexers, the spherical lenses and the graded index type rod lenses have to be provided in pairs, respectively. Besides, adjustment for alignment of the optical axes of the individual components is obviously very troublesome, involving difficulties in implementing the optical transmission system with low loss and high reliability at reasonable costs. Furthermore, reflected light rays from the various devices or elements will possibly be re-injected into the LD, bringing about changes in the longitudinal mode which in turn will produce crosstalk noise. To evade such unwanted phenomenon, corresponding measures must be taken against the light reflections by the various devices, which means that the system structure will then be considerably complicated.
FIG. 2 shows a typical structure of the hitherto known optical multiplexer corresponding to the one denoted by the numerals 6 and 6' in FIG. 1. Referring to FIG. 2, the light of wavelength .lambda..sub.1 traveling through the optical fiber 4 in the direction indicated by an arrow 5 is applied to a rod lens 17 and hence to a pentagonal prism 21 in which the light is reflected by a dielectric mirror 19 to a dielectric thin-film filter 20 at which the light is again reflected toward a rod lens 18. The light signal leaving the rod lens 18 travels through the optical fiber 7 in the direction indicated by an arrow 8. On the other hand, light of wavelength .lambda..sub.2 propagates through the optical fiber 7 in the direction indicated by an arrow 8' and passes through a dielectric thin-film filter 20 to be transmitted through a rod lens 16 to the optical fiber 9' in which the light signal travels in the direction indicated by an arrow 10'. In this way, the light signal of wavelength .lambda..sub.1 and the light signal of wavelength .lambda..sub.2 are separated or demultiplexed from each other. However, the optical multiplexer shown in FIG. 2 requires a great number of optical elements and a complex structure, presenting an obstacle to the realization at low costs and with high reliability. Further, because of the single-mode transmission, the core diameter of the optical fiber has to be smaller than 10 .mu.m, whereby extremely strict accuracy requirements are imposed with regard to the positioning of the optical fibers. As the loss of the optical multiplexer itself as well as loss involved in the couplings between the LD and the lens, between the lens and the optical fiber and between the optical fiber and the optical multiplexer is increased, the transmission distance is correspondingly shortened, making it difficult to realize a long distance transmission. For the reasons described above, the two-way optical transmission system in which a single-mode optical fiber is used is not yet realized for practical applications. By the way, in the one-way transmission system, the transmission distance is only around 5 km.